1. Technical Field
This invention relates to certain polyacetal compositions which are characterized by improved stability and processing due to the inclusion therein of microcrystalline or fibrous cellulose, both of which are derived from naturally occurring cellulose and are non-melting at the temperature at which polyacetal is melt processed.
Polyacetal, also commonly referred to as polyoxymethylene, compositions are generally understood to include compositions based on homopolymers of formaldehyde, the terminal groups of which are end-capped by esterification or etherification, as well as copolymers of formaldehyde or of cyclic oligomers of formaldehyde and other monomers that yield oxyalkylene groups with at least two adjacent carbon atoms in the main chain, the terminal groups of which copolymers can be hydroxyl terminated or can be end-capped by esterification or etherification. The proportion of the comonomers can be up to 20 weight percent. Compositions based on polyacetals of relatively high molecular weight, i.e. 10,000 to 100,000 are useful in preparing semi-finished and finished articles by any of the techniques commonly used with thermoplastic materials, e.g. compression molding, injection molding, extrusion, blow molding, rotational molding, melt spinning, stamping and thermoforming. Finished articles made from such compositions possess desirable physical properties, including high stiffness, strength, low coefficient of friction, and good solvent resistance. However, in certain applications, it would be desirable to improve the thermal stability of the polyacetal composition. By the present invention, the thermal stability of polyacetal is improved through the use of either microcrystalline or fibrous cellulose as a stabilizer, both of which are naturally occurring materials and are non-melting at the temperature at which the polyacetal is melt processed.
2. Background Art
U.S. Pat. No. 4,766,168 discloses hydroxy-containing polymers or oligomers as stabilizers for polyacetal. Naturally occurring materials are not specifically mentioned in this reference as being stabilizers for polyacetal. Neither microcrystalline nor fibrous cellulose, both of which are non-melting at the melt processing temperature of polyacetal, are specifically disclosed within this reference, nor are the advantages obtained from the incorporation of microcrystalline or fibrous cellulose into polyacetal disclosed therein.
U.S. Pat. No. 4,722,662 discloses a process for manufacturing oxymethylene copolymers stabilized against thermal degradation, comprising the heating of the copolymer in an aqueous medium containing water and a disubstituted cellulose ether to hydrolyze unstable oxymethylene ends and then separating said oxymethylene copolymer from said aqueous medium containing a disubstituted cellulose ether. Disubstituted cellulose ethers are known thermoplastics.
U.S. Pat. No. 4,111,887 discloses polyoxymethylene molding compositions exhibiting improved physical properties comprising an admixture of a polyoxymethylene polymer, a fibrous reinforcement which can include cellulosic fiber, and a polycarbodiimide.
U.S. Pat. No. 3,406,129 discloses melt blends of moldable cellulose polymer having free hydroxyl groups with up to 50% of acetal polymer and U.S. Pat. No. 3,406,130 discloses colloidal dispersions of such blends with certain solvents for the cellulose polymer, which compositions are alleged to have improved melt strength and elongation. U.S. Pat. No. 3,406,129 specifically teaches that the use of greater than 50 weight percent polyacetal is detrimental to the melt blend. Further, the cellulose disclosed in these references is neither microcrystalline nor fibrous cellulose as it is moldable, and therefore, meltable at the melt processing temperature of polyacetal.
While some of the reference discussed above disclose incorporating certain particular cellulosics into polyacetal compositions, none disclose the specific types of cellulose used in the compositions of the present invention, nor do any disclose the unexpectedly improved stability in polyacetal compositions that results from the incorporation therein of said types of cellulose.